1. Field of the Invention
The present invention relates to detectors for use in digital receivers. The invention was developed by paying specific attention to the possible use in third generation (3G) UTRA TDD mode systems for downlink detection in mobile equipment. UTRA and TDD are widely used acronyms for Universal Terrestrial Radio Access and Time Division Duplex, respectively.
2. Description of the Related Art
In Code Division Multiple Access (CDMA) system several independent users are simultaneously active in the same frequency band, and they are only separated by different user-specific signature sequences. A set of orthogonal code sequences of variable length (Orthogonal Variable Spreading Factor or OVSF) is used allowing a large flexibility in data transmission rate: see, for instance “The TD-CDMA based UTRA-TDD Mode”, by M. Haardt, A. Klein, R. Koehn, S. Oestreich, M. Purat, V. Sommer, T. Ulrich, IEEE Journal on Sel. Areas in Comm., vol. 18, n. 8, August 2000, pp. 1375-1385, which is incorporated herein by reference in its entirety.
Some efforts have been devoted to investigating receivers for multirate CDMA transmission: see, e.g., “Linear receivers for the DS-CDMA Downlink Exploiting Orthogonality of Spreading Sequences”, by I. Gauri, D. Slock, 32nd Asilomar Conf. on Sig., Syst. and Comp., November 1998; “Low Complexity Group Detectors for Multirate Transmission in TD-CDMA 3G Systems”, by P. Castoldi and H. Kobayashi, Wireless Broadband Symposium (Globecom 2000) San Francisco, 27 Nov. 2000; “Comparison of Maximum Likelihood-based Detection of Two Multi-rate Access Schemes for CDMA Signals”, by U. Mitra, IEEE Trans. on Commun, January 1999; “Interference considerations for the Time Division Duplex Mode of the UMTS Terrestrial Radio Access”, by H. Holma, S. Heikkinen, O.-A. Lehtinen and A. Toskala, Journal on Sel. Areas in Comm., vol.18, n. 8, August 2000, pp. 1386-1393, which are each incorporated herein by reference in their entireties.
Some algorithms have been designed especially for uplink signal detection. These algorithms are based on the zero forcing (ZF) and the minimum mean-square-error (MMSE) criteria as witnessed, e.g., by “Zero Forcing and Minimum Mean-Square-Error Equalization for Multiuser Detection in Code Division Multiple-Access Channels”, by A. Klein, G. Kaleh, and P. Baier, IEEE Trans. on Vehic. Technology, vol. 45, n. 2, May 1996, which is incorporated herein by reference in its entirety. Basically, these solutions are based on channel equalization followed by a code-matched filter, which permits multi-path diversity to be exploited for the radio channel.
The solutions indicated in the foregoing suffer to a lesser or greater extent from a number of drawbacks, e.g.:                a system model that assumes a fixed data rate or an equally fixed spreading factor;        implementation as a fixed block multi-user detection scheme (see, e.g., the paper by A. Klein et al. already cited in the foregoing or “Data Detection Algorithms especially designed for the downlink of CDMA Mobile Radio Receivers”, by A. Klein, Vehicular Technology Conference (VTC '97), Phoenix, Ariz., pp. 203-207, May 1997), which is incorporated herein by reference in its entirety, that is computationally prohibitive even for burst transmission with short data fields;        inter-cell interference, due to the neighboring cells, is not rejected and is only modeled as non-white gaussian noise;        generally, the multiple access interference (MAI) component for a mobile user equipment (UE) is not considered as structured: each mobile has only knowledge of its own signatures and has no information about interfering users in the same cell.        